Many reservoirs of natural gas contain hydrogen sulfide and carbon dioxide which are acid gases which can be extremely corrosive when combined with each other and water. Natural gas containing such acid or sour gases must be purified (or "sweetened") to remove or decrease the concentration of such gases prior to the purified natural gas ("sweet gas") being forwarded to consumer, industrial and other markets.
The most commonly-practised process technology for acid gas removal is the absorption of the gases from the natural gas stream by a regenerable absorbing solution in a gas processing plant. In such procedures, a regenerable absorbing solution is passed in countercurrent contact with the natural gas stream to absorb the H.sub.2 S and CO.sub.2, as well as other sulphur compounds, from the natural gas stream, thereby reducing their concentration in the natural gas stream and purifying the natural gas stream.
The acid gas laden solution then is regenerated by steam stripping at elevated temperature and the regenerated solution is cooled and recirculated back to the natural gas contacting stage. Acid gases stripped from the solution in the regeneration step are vented from the gas processing plant for further processing. The present invention is concerned with a manner of processing such gas streams.
Chemicals that are commonly employed in such procedures include amines, esters and similar regenerable materials in which the acid gases may be absorbed. The most commonly-employed amines for this procedure include monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and diethylmonoethanolamine.
The present invention provides a novel procedure for treatment of the hydrogen sulfide-containing gas stream from a natural gas sweetening operation as described above. As described in more detail below, the process of the invention includes a step of reacting hydrogen sulfide and sulfur dioxide to form sulfur (sometimes termed the Claus reaction) in an aqueous medium. The process described herein is applicable to other gas streams which contain hydrogen sulfide, including Claus process tail gas streams and industrial flue gas streams.
There have been prior proposals of which the inventors are aware for reacting hydrogen sulfide and sulfur dioxide in an aqueous medium.
U.S. Pat. No. 1,925,198 describes contacting a hydrogen sulfide containing gas stream with a counterflowing aqueous solution of sulfur dioxide in a bubble tower to form sulfur. The sulfur dioxide solution is formed by burning sulfur to form sulfur dioxide and then absorbing the sulfur dioxide in water. Operating conditions of eight atmospheres at 40.degree. C. are suggested for the tower, with a steam coil provided at the lower end of the tower to heat the sulfur produced in the tower above its melting point for removal from the tower.
U.S. Pat. No. 2,043,084 describes a procedure in which hydrogen sulfide is "beaten" into a bath of aqueous sulfur dioxide by violent agitation to cause reaction to form sulfur. A suspension of elemental solid sulfur in aqueous medium is withdrawn from the bath for separation of the solid sulfur.
U.S. Pat. No. 2,534,063 describes feeding hydrogen sulfide and sulfur dioxide gases to a pair of reaction tanks through which the gases are successively passed to react to form sulfur. Each reaction tank contains an aqueous acid solution, such as sulfuric acid, hydrochloric acid, phosphoric acid or acetic acid, at relatively high concentration, for example, 25 to 50% sulfuric acid, at relatively ambient temperatures. Solid sulfur product is settled out in a separator unit and the solid sulfur is withdrawn from the separator unit.
U.S. Pat. No. 4,855,124 describes the introduction of hydrogen sulfide and sulfur dioxide simultaneously into a water medium in the absence of any additive and pressurized at a pressure of at least 10 kg/cm.sup.2 gauge (at least about 10 atmospheres gauge or 1100 kPa absolute) with no mechanical agitation at a pH of below 2, specifically 1.5 to 0.1, to form sulfur.
As will be apparent from the description of the present invention which follows, the present invention is concerned with a procedure for effecting reaction of hydrogen sulfide and sulfur dioxide in aqueous medium which is quite different from the procedures described in the prior art.